A railroad vehicle is generally provided with a roof structure which constitutes the top of a vehicle body. In terms of restrictions of construction gauges and exterior designs, structures have been known, where an air conditioner or the like can be mounted by lowering the roof structure so that it is recessed toward a vehicle cabin from the external surface of the roof structure (refer to Patent Document 1). Hereinafter, the structural part which is lowered from other parts of the roof structure is referred to as a “low roof structure.” The low roof structure disclosed in Patent Document 1 is made of an extruded aluminum alloy, which is comprised of a flat part on which the air conditioner is placed and rising parts which rise from both sides of the flat part in a width direction to join to side structures. The flat part is joined to the respective rising parts by welding or friction stir welding. Thus, in Patent Document 1, a reduction in strength and a reduction in a noise generation caused by the joining parts can be prevented, while facilitating an installation work.